1. Field of the Invention
This invention relates to Raman spectroscopy and is directed more particularly to a Raman spectroscopy system and method including an improved optical probe assembly, and an improved specimen holder for retaining a specimen for examination by a Raman spectroscopy system.
2. Description of the Prior Art
Raman spectroscopy is based upon the Raman effect which may be described as the scattering of light from a gas, liquid or solid with a shift in wavelength from that of the usually monochromatic incident radiation.
When a transparent medium is irradiated with an intense source of monochromatic light, and the scattered radiation is examined spectroscopically, not only is light of the exciting frequency, nO, observed (Rayleigh scattering), but also some weaker bands of shifted frequency are detected (FIG. 1). Moreover, while most of the shifted bands are of lower frequency n0−ni, there are some at higher frequency, n0+ni. By analogy to fluorescence spectrometry, the former are called Stokes bands 10 and the latter anti-Stokes bands 12. The Stokes and anti-Stokes bands 10, 12 are equally displaced about a Rayleigh band 14; however, the intensity of the anti-Stokes bands 12 is much weaker than the Stokes bands 10 and they are seldom observed.
If the polarizability of a molecule changes as it rotates or vibrates, incident radiation of frequency n0, according to classical theory, should produce scattered radiation, the most intense part of which has unchanged frequency. This is referred to as Rayleigh scattering.
In addition, there typically are Stokes and anti-Stokes lines 10, 12 of much lesser intensity and of frequencies n0+/−ni, respectively, where ni is a molecular frequency of rotation or vibration. The anti-Stokes lines 12 are always many times less intense than the Stokes lines 10. This fact is satisfactorily explained by the quantum mechanical theory of the Raman effect.
The vibrational Raman effect is especially useful in studying the structure of the polyatomic molecule. If such a molecule contains N atoms it can be shown that there will be 3N−6 fundamental vibrational modes of motion only (3N−5 if the molecule is a linear one). Those which are accompanied by a change in electric moment can be observed experimentally in the infrared. The remaining ones, if occurring with a change in polarizability, are observable in the Raman effect. Thus, both kinds of spectroscopic measurements are usually required in a complete study of a given molecule.
Like infrared spectrometry, Raman spectrometry is a method of determining modes of molecular motion, especially the vibrations, and their use in analysis is based on the specificity of such vibrations. The methods are predominantly applicable to the qualitative and quantitative analysis of covalently bonded molecules, rather than to ionic structures. Nevertheless, they can give information about the lattice structure of ionic molecules in the crystalline state and about the internal covalent structure of complex ions and the ligand structure of coordination compounds both in the solid state and in solution.
Both the Raman and the infrared spectrums yield a partial description of the internal vibrational motion of the molecule in terms of the normal vibrations of the constituent atoms. Neither type of spectrum alone gives a complete description of the pattern of molecular vibration, and, by analysis of the difference between the Raman and the infrared spectrum, additional information about the molecular structure can sometimes be inferred.
Physical chemists have made extremely effective use of such comparisons in the elucidation of the finer structural details of small symmetrical molecules, such as methane and benzene, but the mathematical techniques of vibrational analysis are not yet sufficiently developed to permit the extension of these differential studies to the Raman and infrared spectra of the more complex molecules that constitute the main body of both organic and inorganic chemistry.
The analytical geologist can use Raman and infrared spectra in two ways. At the purely empirical level, they provide “fingerprints” of the molecular structure and, as such, permit the qualitative analysis of individual compounds, either by direct comparison of the spectra of the known and unknown materials run consecutively, or by comparison of the spectrum of the unknown compound with catalogs of reference spectra. By comparisons among the spectra of large numbers of compounds of known structure, it has been possible to recognize, at specific positions in the spectrum, bands which can be identified as “characteristic group frequencies” associated with the presence of localized units of molecular structure in the molecule, such as methyl, carbonyl, or hydroxyl groups. Many of these group frequencies differ in the Raman and infrared spectra.
Thus, Raman spectroscopy is a viable technique for identifying and characterizing a vast array of compounds and materials. Applications of Raman spectroscopy are far reaching in both the scientific and the industrial arenas. Industrial areas of use include medical, biotechnology, pharmaceuticals, security, and geology. Recent technology advancements are enabling increasing application reach through a reduction in cost and size. Portable units (FIG. 2) are becoming available for out of lab uses in the measurement and identification of powders, pills, and liquids.
A persistent problem in the design of such systems is the delivery and collection of laser light and the Raman signature from the specimen. It is often desirable to have a flexible light guide for light delivery and collection. It is also often a requirement that such delivery and collection light guide be flexible, rugged, and compact in size.
Traditional optical fibers have been used for light delivery to the specimen. However, the high intensity of optical power and the non-linear coefficients of the optical fiber's refractive index cause the generation of a background Raman signal. As illustrated in FIG. 3, the presence of this background signal reduces the signal to noise of the Raman signature measurement if a single traditional optical fiber, as illustrated in FIG. 2, is used for both light delivery and collection. Thus, two or more light guides 24A and 24B are typically used, as shown in FIG. 4.
Raman spectroscopy works by launching from a laser source 16 a laser light of a particular wavelength, typically in the visible or near infrared, at a specimen 28 and collecting to an optical spectrum analyzer 18 light which has been Stokes shifted to longer wavelengths through vibrational mode interactions. By studying the position in energy of these shifted peaks, a signature of a particular material is obtained.
Traditional optical fibers introduce amorphous glass into the optical path. The amorphous nature of the glass fiber causes a broad Raman peak 38 (FIG. 3) which is collected and superimposed onto the Raman signature of the specimen. The result is a decrease in sensitivity and hence material selectivity.
Another persistent problem with Raman spectroscopy is the interference of the inherent Raman signature of the typical glass vial in which the specimen is contained with the Raman signature of the specimen being tested.
There is accordingly a need for a Raman spectroscopy system and method which includes an optical probe assembly which exhibits a low Raman cross section within the optical probe, thus reducing contamination of the specimen's Raman signature with any background Raman signal generated in the probe.
There is further a need for such a system in which the optical probe is compact, flexible, and rugged.
There is still further a need for a Raman spectroscopy system and method in which amorphous glass does not obstruct the optical path, with resulting improved sensitivity of the system.
There is yet further a need for a specimen holder for conducting Raman spectroscopy in which the Raman signature of the material of the container is configured to avoid interference with the Raman signature of the specimen.